In conventional cylindrical drying apparatus the jacket of the drying cylinder is generally heated by steam which is supplied under pressure to the interior of the cylinder. Heating is thus effected such that the steam condenses in the cylinder and thereby emits heat therein. After the steam has condensed, however, the condensate must be removed from the cylinder. It is thus desirable that as much steam as possible be condensed in the cylinder in connection with such heating apparatus. This therefore implies that the amount of steam removed from the cylinder along with the condensate be minimized.
Because of the centrifugal force produced by rotation of the drying cylinder, the condensate is collected along the inner surface of the cylinder. In order to discharge the condensate from the cylinder, a pipe such as a siphon pipe is usually arranged radially within the cylinder. The siphon pipe thus communicates with a collecting pipe which is generally provided centrally within the cylinder and which sealingly extends out of the cylinder to a collecting vessel. The condensate is thus discharged by means of a pressure difference maintained between the steam space within the cylinder and this collecting vessel.
Such siphon pipes have thus either been stationary, i.e. non-rotary with the cylinder, or may rotate with the cylinder by being clamped therein. In the former case the medium flowing within the radial pipe would not be effected by any centrifugal force. By thus designing the end of the siphon pipe in a suitable manner, the kinetic energy of the rotating condensate at the stationary end point of the siphon pipe can be utilized for discharging the condensate. This type of design, however, has proved much less suitable in connection with high speed drying machines.
The siphon pipe is thus most usually designed to rotate with the cylinder, and in that case the resistance arising in transporting the medium through the radially extending pipe against the centrifugal force therein must be taken into consideration. The siphon pipe thus employed at present is normally designed in order to render this pressure resistance as low as possible. This may be accomplished by admixing from about 10 to 25% of steam with the condensate so that the two-phase medium including steam and water has a much lower density than if the medium consisted of the water alone. Such a solution is, however, disadvantageous in that a great amount of overflowing steam must then be separated for re-use. The latter is accomplished in so-called cascade systems employing a plurality of successively decreasing groups of condensers connected in series until a small residual amount of steam (i.e. between about 1 and 5%) is condensed in a particular condenser. It is also inherent in the function of such cascade systems that the flows and pressures are tied to correct flow of steam and condensate as well as steam pressures and temperatures, which are not always adapted to drying processes in such drying machines.
The use of such siphon pipes, however, involves several disadvantages. It is difficult to obtain sufficient structural strength therein, and furthermore the location of the pipe within the cylinder renders access to the siphon pipe for purposes of cleaning extremely difficult. One such system which employs such siphon pipes is shown in Swedish Pat. No. 74255.
Another method for removing condensate from such rotating vessels is shown in British Pat. No. 956,588. That patent employs a rotating vessel which has an internal surface which has an increasing diameter as one moves from the center of the vessel to its ends. In this manner liquid within the vessel is caused to flow outwardly to its ends, and means are thus provided for discharging same. In particular, however, in FIG. 5 of this British patent, a device used in connection with the internal shape of the drum is shown including nozzles around the ends of the drum cooperating with a stationary annular duct coaxial with the drum and having a continuous opening facing the drum to receive liquid emitted from the nozzles. In particular, the nozzle bores are preferably reduced towards their outer end to maintain a pressure differential between the interior and exterior of the drum. Apart from the required unique shape of the drum employed in connection with this patent, it further does not overcome prior difficulties such as difficulties in cleaning the apparatus, and further difficulties are raised by this apparatus, including the annular duct for condensate collection, and the fact that it is open to the atmosphere.